As a model to study regulated RNA processing of pre-mRNAs in eukaryotic cells, we are studying viral cis elements and host trans-acting factors required for proper RNA processing and replication of Rous sarcoma virus (RSV). RSV primary transcripts are spliced and polyadenylated by the host RNA processing machineries to generate env/src mRNA, but the process must be controlled since the virus requires that a majority (75%) of the RNA remain unspliced to serve as mRNA for the gag-pol proteins and as genomic RNA for progeny virions. A novel RNA element (the NRS) that binds numerous splicing factors is required for accumulation of appropriate unspliced RNA levels and also for proper polyadenylation. NRS mutations can lead to replication defects due to oversplicing and to pathogenesis from the increased production of poly(A) read-through transcripts that play a role in oncogenesis via insertional activation. Aim I will investigate the role of various factors in NRS splicing inhibition and explore mechanisms of inhibition involving novel, inappropriate snRNP interactions. In Aim II, cis elements and trans factors that mediate efficient snRNP binding to the NRS will be studied. In Aim lll, the role of the NRS in promoting efficient polyadenylation will be investigated. In Aim IV, the function of a second splicing suppressor (SSS) associated with the src 3' splice site will be studied. These Aims are significant in that they will enhance our understanding of control of critical RNA processing steps required for viral replication and pathogenesis, and the RSV system provides a powerful tool for dissecting novel cellular mechanisms of RNA processing regulation.